A wave is all kinds of periodic oscillation that occurs on the surface of the sea. The waves occur at sea, caused by wind, storms, tsunamis, forces of attraction, gravitation, etc. However, from all of them, the wind is the one that generates the most common waves and with higher energy density.
The greater the height of the waves, the greater is the amount of energy that can be extracted from them. The height of the waves depends on three wind parameters: the speed, its persistence over time, and the stability of its direction.
In the state of art there have been developed a plurality of devices for capturing this large amount of wave energy. The devices for taking advantage of the energy stored in the waves must capture the kinetic and/or potential energy of the waves and convert it into another form of useful energy. However, there are some problems in the design of such devices. For example, the irregularity in the amplitude, phase and direction of the waves makes it difficult to obtain the maximum efficiency of the apparatus over the full range of frequencies of excitation, while extreme weather conditions are not favourable either.
Thus, the devices for extracting the energy of the waves are classified according to different criteria. Depending on the relative position of the devices with respect to the coast, there can be distinguished devices located on the coast, close to the coast and on the high seas. Depending on their ability to extract energy, they can be small, medium or large capacity devices. If they are classified by their geometry and position relative to the waves, there can be identified totalizers or terminators, with the main axis parallel to the waves, i.e., perpendicular to the direction in which the waves move, extracting the energy of the waves at once; the attenuators are devices the main axis of which is parallel to the direction of motion of the waves, extracting the energy progressively; and point collectors, which are isolated devices with reduced dimensions that take advantage of the concentration and convergence of the waves. By their relative motion with respect to the waves, devices with structure fixed to the coast or to the sea bottom and floating or semi-submerged apparatuses can be identified.
Fixed structure devices are anchored to the sea bottom or the coast, such that the main structure does not move with the sea, however, these devices have elements that can move with respect to the fixed structure when the waves act on them, converting the wave energy into mechanical energy, which is transformed into another form of useful energy. These devices are easier to manufacture and maintain than floating devices, but they have other drawbacks such as the small number of coastal locations wherein they can be installed or the lower capture of energy than floating devices that are installed away from the coast.
Devices based on oscillating water columns consist of a hollow, partially submerged structure that is open to the sea at the bottom. This structure contains an air column at the upper part of a water column. When the waves act on the apparatus they cause the water column to go up and down, with which the air column is compressed and decompressed alternately. In other words, it takes advantage of the principle of resonant cavity. If this trapped air is allowed to flow to and from the atmosphere through the blades of a turbine, mechanical energy can be extracted from the system and used to generate electricity through a generator mechanically attached to the turbine. Generally used turbines are the so-called Wells turbines, which receive the name of their inventor, Professor Alan Wells. These turbines have the property of rotating in the same direction regardless of the direction in which the air passes through their blades of symmetrical profile.